Saturday, December 31, 2016

December Snowfall

Here's a quick update on yesterday's post: snowfall for December is now up to 32.9" in Fairbanks, which is the 4th highest on record (1930-present).  Nearly half of that fell in the last three days.  The airport recorded wind gusts as high as 52 mph early this morning, and sustained winds of 33 mph were measured; this sustained wind speed is the highest since February 2011.  It's also the highest for early-mid winter (pre-February) since 1970.

This morning's sounding showed an extraordinarily strong westerly flow aloft, with 850mb winds of 75 knots (86 mph).  This ties the all-time record for strongest 850mb wind speed in Fairbanks (1948-present); this has been an extreme and historic winter storm.

Returning to the topic of the relationship between snowfall and North Pacific sea surface temperature patterns, the following chart illustrates the unusual nature of this month's heavy snowfall.  The horizontal axis shows the difference between the PDO and NPM index values (PDO minus NPM), and December's snowfall is shown on the vertical axis.  When the PDO is positive and the NPM is negative, as they are now, it is unusual to have significantly above-normal snowfall.  Note that I calculated the linear trend line after transforming the snowfall data into an approximately normal distribution, because ordinary least-squares regression is not suitable for a skewed distribution like we have here (it weights the outliers too heavily).

Friday, December 30, 2016

Major Storm

A sprawling storm system stretching from far eastern Russia to interior and northern Alaska brought heavy snow to the Fairbanks area last night and is now producing strong winds (gusting to 50mph at the airport in the past couple of hours).  Here are preliminary snowfall totals from the NWS as of this morning:

Public Information Statement
National Weather Service Fairbanks AK
822 AM AKST Fri Dec 30 2016

...Snowfall Reports...

Location                     Amount    Time/Date       Lat/Lon              

...Middle Tanana Valley...
3 S Fox                      9.4 in    0500 AM 12/30   64.92N/147.62W       
20 NW Fox                    9.0 in    0500 AM 12/30   65.18N/148.07W       
1 NW Fairbanks               8.0 in    1100 PM 12/29   64.85N/147.69W       
2 NW College                 8.0 in    1000 PM 12/29   64.89N/147.88W       
2 N College                  8.0 in    0900 PM 12/29   64.90N/147.82W       
3 SSW Fox                    8.0 in    1000 AM 12/29   64.92N/147.65W       
4 NNW Fairbanks              7.6 in    1000 AM 12/29   64.89N/147.68W       
3 N Fairbanks                5.9 in    0830 PM 12/29   64.88N/147.65W       
Fox                          5.8 in    0845 PM 12/29   64.96N/147.63W       
2 W Two Rivers               5.5 in    1000 PM 12/29   64.87N/147.17W       
Two Rivers                   5.5 in    1000 PM 12/29   64.87N/147.09W       
North Pole                   4.0 in    0800 PM 12/29   64.75N/147.36W       
4 SSW College                12.9 in   0400 AM 12/30   64.82N/147.88W       
3 SW College                 11.8 in   0530 AM 12/30   64.84N/147.90W       
3 SW College                 11.8 in   0200 AM 12/30   64.84N/147.90W       
2 NNW College                11.7 in   0617 AM 12/30   64.89N/147.85W       
12 SW Ester                  11.0 in   0300 AM 12/30   64.74N/148.32W       
3 N Fairbanks                10.4 in   0530 AM 12/30   64.88N/147.65W       
1 E College                  10.0 in   0123 AM 12/30   64.87N/147.79W       

...Lower Koyukuk and Middle Yukon Valleys...
Kaltag                       9.0 in    0600 PM 12/29   64.34N/158.69W       

...Central Interior...
Nenana                       10.0 in   0300 AM 12/30   64.54N/149.09W

The surface and 500mb charts show the powerful southwesterly flow that is typical of heavy snow events in Fairbanks.  Here are the maps from 3pm AKST yesterday (click to enlarge):

and from 3am today:

The intensity of the flow into the interior is very pronounced and explains both the abundant moisture (snowfall) and windy conditions.  This afternoon's sounding from Fairbanks airport measured a wind speed of 47mph at 925 mb (~2000 feet above ground), which is the strongest in more than 6 years during the winter season.  The sounding (see below) also shows a remarkably strong lapse rate in the lower troposphere, meaning that the temperature profile is highly favorable for vertical mixing and downward transport of momentum.  It's very rare to see this kind of temperature profile in mid-winter: only one December day in Fairbanks history had a more unstable sounding from the surface to 700mb; that day was December 12, 1972, when 13.9" of snow fell over a 3-day period.

December 2016 is now in 5th place for total snowfall (1930-present) and will probably reach 3rd place by tomorrow.

Tuesday, December 27, 2016

Surprising Snowfall

In a change of scene that is no doubt refreshing for some, Fairbanks has seen a significant amount of snow in the past several weeks.  The total so far this month is 16.9", compared to a normal accumulation of just under 10" for the full month of December.  With more snow in the forecast, it looks like this month will beat the 18.6" of December 2012 and become the snowiest December since 1992 (28.5").

Here's a chart of the season-to-date snow accumulation (green line) compared to normal (brown line), recent years, and the historical extremes.  I created the chart on the xmACIS2 interface, which provides a wide variety of handy analysis options.  Fairbanks is still in the lower tercile for season-to-date snowfall owing to the very dry start to winter.

A couple of weeks ago I mentioned that the PDO was still moderately positive, which was interesting in view of the notable cold spell that managed to envelop most of the state earlier this month.  The positive PDO phase also tends to be associated with below-normal snowfall in interior Alaska, as we saw in the past couple of winters; and so the current snowy spell does not reflect the PDO influence.  Furthermore, the NPM (North Pacific Mode) is strongly negative at present, and this too is normally associated with dry weather in winter.

The chart below shows the rank correlation coefficient by month between snowfall and the PDO and NPM index values.  For the November-March seasonal average, the NPM actually has a slightly stronger correlation with Fairbanks snowfall than the PDO.  Given that the current NPM index is among the most negative on record, and the PDO is also positive, it's quite surprising to see a snowy weather pattern emerge in Fairbanks.

Here's a recent SST analysis, showing North Pacific temperature anomaly patterns that are characteristic of the positive PDO and especially the negative NPM:

See these earlier posts for more perspective on the PDO and NPM and their impacts:

Thursday, December 22, 2016

River Ice, Satellite Data, and Solstice Sun

I've been busy lately, but here are a few miscellaneous items that may or may not interest a few readers.  First - the webcam from Dawson, Yukon, is still showing a rather considerable amount of open water on the Yukon River more than two months after ice began forming.  This is despite daily mean temperatures in the -30s Fahrenheit for a week earlier this month.  The incomplete freeze-up could reflect warmer than normal temperatures back in November, although it's not necessarily that unusual to see open leads on northern rivers even in the depths of winter.  Those more familiar with the Yukon River at Dawson would be able to say how unusual this particular situation is.  [Update: this article gives some perspective:]

The Tanana River at Nenana finally froze over completely in the past week or so, according to webcam views.  The first image below, from December 15, shows the last vestiges of the central channel that was last to freeze over.  Yesterday's image (second below) showed a more satisfactory scene.

On another note, I've recently figured out how to decode the land surface temperature estimates from the Suomi NPP satellite.  This satellite orbits the earth about 14 times each day and measures (among other things) emitted infrared radiation, leading to an estimate of surface temperatures when clouds are absent.  The data are available at a resolution of 750m, which allows for some really nice spatial detail.

I thought it would be interesting to examine the minimum surface temperature estimates from the recent cold spell, so I extracted the lowest value produced by the various scans available each day from December 6-10; see below for maps.  Note that there are some obvious artifacts in the data (e.g. on the 10th), and there is missing data in some areas owing to cloud and fog (including probably ice fog in the coldest locations); but overall the data look quite good.  The very coldest temperatures on these days (for this map domain) were found over the Old Crow Flats of northern Yukon.

For comparison, the observed daily minimum temperatures at the 3 locations marked on the maps were as follows (Fairbanks, Chalkyitsik RAWS, and Chicken respectively).  A detailed long-term investigation of the quality of the satellite estimates over Alaska would be most interesting.

Dec 6-29°F-51°F-42°F
Dec 7-29°F-52°F-27°F
Dec 8-25°F-51°F-18°F
Dec 9-28°F-49°F-42°F
Dec 10-26°F-32°F-52°F

Finally, the FAA webcam at Ambler yesterday provided an opportunity to see the brief sunrise that occurs on the winter solstice even above the Arctic Circle (owing to atmospheric refraction).  Ambler is about 35 miles north of the Arctic Circle but still experiences over an hour of full daylight even on the shortest day of the year.  One has to go about 20 miles farther north to have no sunrise on December 21.

Wednesday, December 21, 2016

Winter Solstice in Fairbanks

Hi, Rick T. here, and happy winter solstice to you and yours. Outside, winter solstice is often thought of as the first day or winter, but in Alaska, this day is a big deal because of the solar aspect. After all, we've had "winter" weather for months, and no sourdough would confuse the dim of late December with blinding sunshine of late-February. Symbolically, winter solstice is the darkest day of the year, though in fact several days around solstice are effectively the same. Here in Fairbanks, the sun will be above the horizon for 3 hours and 43 minutes, the brightest stars are visible on a clear solstice morning until almost 10am, and reappear by 345pm. Early in the 20th century, winter solstice fell on December 22 more often than not, becoming more common on December 21 as the century progressed (as reckoned as the day of occurrence in Fairbanks based on the time zone in use at the time). This is due in part to the slow precession of the earth's axial tilt and in part to the multiple time zone changes.

So, just for fun, here's a plot of daily temperatures on winter solstice day in Fairbanks since 1904. Some solstice factoids: highest temperature was 42F in 1939; lowest temperature -49F in 1961. Six winter solstices have highs above freezing, and ten have had lows of 40 below or lower. [Note: I've put up a similar version of the plot below on twitter, #akwx]

Thursday, December 15, 2016

Cold Spell Ends

After nearly two weeks of significantly colder than normal temperatures in Fairbanks, clouds finally rolled in this morning and produced warming at the surface.  The temperature rose from -21°F at midnight to +9°F at 3pm as infrared radiation from the clouds above warmed the air at the surface and thereby weakened the surface-based inversion.

The chart below (click to enlarge) shows the hourly temperature data from the airport in the past 19 days, along with the 850mb and 925mb temperatures as well as the sea-level pressure (MSLP).  The strong inversion began on the 3rd when the pressure rose up through 1030mb, and the inversion weakened today as the pressure dropped back below 1030mb.  Of course the pressure itself is not directly responsible for the temperature changes, but it's correlated with the cloud cover: high pressure conditions tend to be clear and calm, allowing for large net radiation losses from the surface.

With the temperature staying below 0°F for 17 consecutive days and below -13°F (-25°C) for 11 days, this cold spell marks quite a change from recent warm winters; the last comparable cold period was in early December 2012.

It's interesting to note that the PDO phase is still moderately positive; its influence has certainly not been felt lately over interior Alaska.

Here's a chart of recent daily high temperatures, which have been more unusually cold than daily low temperatures.  Prior to today, 27 consecutive days had a high temperature below normal; that's the most since the very cold spring of 2013.  It will be interesting to see if the exceptional and persistent warmth that began so dramatically in May 2013 has ended for now, or if warmth will soon return.

Monday, December 12, 2016

Arctic Update

Last month I introduced a set of 19 climate observing sites around the periphery of the Arctic Ocean that allows us to get a reasonable picture of basin-wide temperature variations in the context of historical observations stretching back several decades.  This is rather useful because reanalysis estimates of temperature are subject to model error and bias, including sometimes errors in the long-term trends.  Of course measurements at surface observing sites have their own quality concerns including sometimes significant urbanization influences (e.g. at Utqiaġvik/Barrow).

The chart below shows the November mean temperature for each year since 1971, and just as for October the warmth this year stands out as unprecedented in the modern era.  The November mean anomaly (departure from normal) of +6.7°C was nearly 3°C higher than the previous warmest November (2012, +3.9°C).  This November's anomaly was also greater than October's anomaly (+5.6°C) and higher than that of January 2016 (+6.5°C); this makes November 2016 the warmest calendar month relative to normal in the data set.

The daily temperature analysis (see below) shows that peaks of basin-wide warmth occurred at the beginning of the month and around the 20th of the month.  By the end of November the anomaly was fading as more normal conditions returned to the Arctic overall, but nevertheless the mean temperature has remained above normal every day.

A couple of the stations (on Russia's central Arctic coast) have become more than 10°C colder than normal in the past couple of weeks, but other sites have remained extremely warm relative to normal; for example, Ambarchik on the coast of the East Siberian Sea reported a daily high temperature well above freezing  (+40°F) last Thursday (December 8).  For Ambarchik last Thursday, the daily mean temperature's departure from normal was an extraordinary +27.1°C (+49°F), which ties the most extreme daily warm anomaly for any of the 19 stations since 1971; the record was previously held by Deadhorse (Alaska) on January 15, 1997 (49°F warmer than normal).

The NSIDC's latest report provides additional context on November's Arctic weather patterns and the state of sea ice.  November's Arctic sea ice extent was the lowest on record for the month, and the monthly ice extent remains more than 3 standard deviations below normal (although not quite as anomalous as during October).

Although not directly related, the ice conditions in the Antarctic are so unusual that they also deserve a mention; the November mean ice extent in the south was by far the most anomalous on record (for any month) at 5.7 standard deviations below the 1981-2010 normal.  This year's conditions are an extreme outlier compared to the climate of the last few decades, as seen in the NSIDC's graphic below.  On a global basis, sea ice extent in November was an astonishing 7.2 standard deviations below the 1981-2010 normal.

Tuesday, December 6, 2016

50 Below Reached

The Chalkyitsik RAWS reported -51°F this morning, which is the lowest observed temperature of the winter so far in Alaska (except for a dubious reading from the Bettles SNOTEL, which seems to have a cold bias).  The Chalkyitsik RAWS is located about half way between Fort Yukon and the village of Chalkyitsik, and is more than 100 feet lower in elevation than the latter.  Climate observations from Chalkyitsik itself were recorded only from 1962 to 1972, and the data are quite incomplete.

Temperatures in Fort Yukon were nearly steady at around -45°F today.  Below is a simple animation of the south-pointing FAA webcam view during the 3 hours of sunshine.  It's interesting to see the apparent fluctuations in the concentration of condensate or particulates in the shallow surface layer.  I'm not quite sure if the haze is composed of smoke or ice crystals, although the airport reported ice crystals in the air for most of the day, and the reported visibility varied from 0.5 to 2.5 miles.  With a strong temperature inversion undoubtedly present in the lowest levels, the haze was trapped very close to the surface - even below the tops of the trees at times, judging from the visual appearance.  A lot of interesting physics goes on in the surface layer of a winter day north of the Arctic Circle.

Monday, December 5, 2016

Cold Spell Persists

It's been quite a while since interior Alaska has seen persistently cold conditions during winter, so the current lengthening cold spell is a notable change of scene.  Including today, Fairbanks has been colder than normal for 17 of the last 18 days.  Here's an update to my chart showing the daily statewide minimum temperature; this is based on 24-hour minimum temperature reports ending at various times of day, so the daily measurement windows are not the same everywhere, but the trend is clear: the colder spots in Alaska have produced -40s for more than a week now.

The coldest area last week was the north-central interior, with the Norutak Lake RAWS reaching -40° or colder for 8 consecutive days ending yesterday.  The coldest conditions have now shifted to the northeast, with the Chalkyitsik RAWS sitting at -48°F this evening and apparently on track to break -50°F tonight.  Last winter no station in Alaska quite reached -50°F, although the Kanuti Lake RAWS got very close.

Environment Canada's surface chart from yesterday afternoon showed strong high pressure over northeastern Alaska, and the Fairbanks sounding measured a cold column with 850mb temperatures well below -20°C.  However, strong warming has been occurring aloft in the past 24 hours as the high pressure produces subsidence; note the difference between the two soundings below (3pm yesterday and 3pm today).

The 1.5-meter temperature measurements from UAF's Poker Flat Research Range show the warm-up nicely; this morning the temperature jumped from -28°F to -7°F in only half an hour.  The Poker Flat site is located near Chatanika on the Steese Highway north of Fairbanks at an elevation of about 700 feet above sea level.

Thursday, December 1, 2016

Lack of Warming in November

A couple of months ago I made a passing reference to the rather noticeable absence of a long-term warming trend in November upper-air temperatures as measured by balloon soundings from Fairbanks.  Based on sounding data from 1950 to present, November mean 850mb temperatures actually show a slight cooling trend, in contrast to the adjacent months of October and December - see the charts below.  Note that Octobers have been unusually warm since 2002, and Decembers have been warmer since the early 1980s, but November mean temperatures show neither of these features.

I've drawn linear trend lines on these charts, but this is just to gain a simple impression of the long-term changes; obviously the changes have not occurred in a linear fashion.

Looking at surface temperatures from Fairbanks over a longer period (1930-2015), we see a similar pattern: temperatures have been warmer in the past 15 years in October, and December has become much warmer over the long haul, but November has seen very little change.

At the risk of boring readers with too many charts, I think it's worth looking also at the Alaska statewide average temperatures from NOAA's climate division data.  It's quite a similar picture here again, although as a whole the state has seen some slight warming in November; this warming has all been in the western half of the state.  I'll make a map of the regional trends when I have more time.

The obvious question, of course, is why has November not warmed in Fairbanks and the eastern interior - not even in the past decade or two.  We might be tempted to pin part of the blame on the PDO phase becoming slightly more negative over the long term (see below), but this hasn't alleviated the strong warming trend in December (and the PDO trends are very similar for the two months).

My working hypothesis for further investigation is that the atmospheric circulation patterns have shifted over the decades so that relatively cooler conditions are now favored in November for interior Alaska; for example, perhaps low pressure is now less common over the Bering Sea in early winter.  This kind of shift could allow a localized region to defy broader hemispheric trends towards warmer conditions; but even if we can show this has occurred, the deeper question of "why" may be difficult to answer without delving into global climate model simulations.  But hopefully I'll be able to report back here with some more answers before too long.